ADC Noise

[gviereck]

We use a STM32F7xx microcontroller with LQFP144 case
Part of the ADC Pins are on the 1-36 pin side.
Unfortunately this is also the side where the Quartz (Pins  OSC_IN and OUT, PC14-OSC32_IN, PC15-OSC32_OUT) and the RMII_CLK are mapped.
This leads to additional noise (app. 10 LSB with peak regions at 500 Hz compared to 5 LSB  noise on other similar filtered sensor signals)  on the pins 36 (ADC123_IN2) and 37 (ADC123_IN3) despite anti-aliasing filter.
Unfortunately we could not find any other pin available on ADC1 and ADC2.
Is this a well known problem that oscillators may disturb ADC's of STM32Fxxx microcontrollers?

[Amel NASRI]

 Hi @gviereck ,

In the errata sheet ES0290, we recognize that some internal noise may impacting the ADC accuracy and 2 workaround options are suggested.

I invite you to review this errata and apply the workarounds then to let us know the results. To enhance the ADC accuracy, don't forget the AN4073 as well.

-Amel

[gviereck]

Hello Amel,
Thanks very much for having answered so quickly.
The annoying additional noise is not a general issue for all ADC pins.
We have 6 sensor signals connected to ADC Pins.
2 of them show additional noise.
They are connected to PA2 (Pin 36 of STM32F756ZG in LQFP-144 case) = ADC123_IN2 and PA3 (Pin Nr 36).
PB0 = Pin 46 = ADC12_IN8 gave better results
 but PC0 = Pin 26 = ADC123_IN10 or PC2 = Pin 26 = ADC123_IN12 gave no better results.

Is this worth trying PC1 or PC3 = Pin 29, too ? 

Layout does not seem to be the problem but more to which ADC pin the sensor signal is connected. 

[ONadr.1]

Do all sensors have the same output impedance and voltage level?

[gviereck]

Yes.
I measured the noise without anything connected to the input stage composed of anti-aliasing filter and voltage adaption circuit.

[SofLit]

Hi @gviereck 

It seems your issue is related to the cross talk phenomena. Please refer to the AN2834 "How to optimize the ADC accuracy in the STM32 MCUs" / section 3.2.11 I/O pin crosstalk.

Solution in the section: 4.2.11 Minimizing I/O pin crosstalk

[gviereck]

Thanks for you contribution.
I wish, you were right.
But the fact is that the sensor tracks do not cross or go along high speed signals. 
Moreover, we have a several layer PCB among which are analog and digital ground planes and supplied voltage (e.g. +3.3V power-supply planes). 

[AScha.3]

1. pin36/37 are not close/next to osz.

ed. sorry, i thought on H7... 16bit.  so on F7xx 12bit adc is a little bit more - about 8mV.

2. 10LSB at 16 bit adc here (ref. 3v3) is about 0,5mV . so to see, what the adc is doing, solder short wire from pin36 to pin31 and cut any >10mm long trace at pin36. THEN you can see, is the reason stray cap/induced spikes or bad chip multiplexer or whatever on chip. And dont forget, to keep the nearby pins "quiet", not switching. Also switching pins on same port mux may produce spikes, if port speed is high (set all ports here to low speed) or high speed signal is coming in there , producing some ringing/overshoot, and some mV spikes are here (via the internal protection circuit/diodes).

3. try with different sampling time (i found best results at 8.5cycles - ymmv); these adc are real fast in the sample/hold inputs, even stray in at 2,4Ghz (wifi or mobile net...) producing effects in 10...20LSB range, or more.

So try with some screen, if possible.

[gviereck]

Hello AScha.3,
Thanks for your contribution that makes sense.
Some more details about our HW and FW implementation:
1) STM32F756 has 3 ADC's with 12bit resolution.
=> 1 LSB corresponds then to 0.8 mV
2) Within our actual FW the sampling frequency is 10 kHz per channel.
3) The LQFP-144 case is a square form.
The side with pin nr's 1-36 receive the 2 quartz (CPU main clock and RTC Clock) and outputs the RMII clock for Ethernet connection.
4) The 2 (more) noisy ADC inputs are also on this side.

5) PA2 = ADC123_IN2 = Pin nr 36 is next to PA1 = RMII_CLK 

6) Connection distance between last filter stage and PA3 = Pin nr 37 is very short (3.3 mm) and is crossing nor getting along no other signal.

6) DSP law: When aliasing occurs the digitalized signal is unrecoverable

[AScha.3]

ok, so for a test: i would disable the RMII clk, this is hi speed and may be the source of some cap. stray in.

and next : the test with a short wire to vssa . (if still "unusual" noise there)

just guessing: rmii clk at pin 35 and adc-in at pin 36 - only a ground trace between the pins could (!) reduce cap. coupling as some screen; better not use a pin so close to a RF clock or similar hi speed signal.